This invention relates to single sideband radio receivers, and it relates more particularly to such receivers which are useful for mobile radiotelephone service.
Mobile radiotelephone signals are subject to rapid deep fading due to the multipath environment in which they are utilized. This is particularly true of such signals when transmissions are at frequencies in the microwave spectrum, i.e., from about 450 megahertz (MHz) up to 10 or 20 gigahertz (GHz). The problem is acute in single sideband (SSB) transmissions because they rely on amplitude variations for conveying information.
Feed forward gain correction affords some relief because it allows the correction to be effected in a timely phase-aligned fashion. The term "feed forward gain correction" here refers to the derivation from the radio channel of a signal to control automatic adjustment of the gain through a part of the receiver downstream, in a signal flow sense, from the point of control signal derivation to modify information signal amplitude in a direction tending to offset the effects of fading experienced in the radio channel. The offset is intended to restore signals to a level that conforms to a predetermined transmission level plan for the radiotelephone system. One example of feed forward gain correction in a single sideband receiver is found in the copending application of K. W. Leland, Ser. No. 065957, filed Aug. 13, 1979, entitled "Single Sideband Receiver with Pilot-Based Feed Forward Correction for Motion-Induced Distortion" now U.S. Pat. No. 4,313,211, and which application is assigned to the same assignee as the present application. In that Leland application, the use of companding was contemplated for suppressing to some extent the noise effects that become noticeable during quiet intervals in speech.
The concept of limiting gain correction was mentioned by M. J. Gans and Y. S. Yeh at page 206 (without disclosed implementation) in Microwave Mobile Communications, edited by W. C. Jakes and published in 1974 by John Wiley and Sons. Gain limiting was achieved in the Leland application by limiting excursions from a predetermined normal pilot signal level in order to prevent large signal level excursions in the audio output that result from quasi-infinite corrections that may otherwise occur when a pilot signal used for control of gain correction fades to a near-zero amplitude level. Such excursions have been found to be annoying to radiotelephone listeners. The limiting effect represented only a small departure from the transmission system level plan and was effective during only deep fade times.
Nevertheless, difficulties persist in that limited, but still large, correction signals occurring during a period of quiet audio can cause annoying enhancement of noise and cochannel interference which appear during a fade on the desired signal which is being corrected. Prior attampts to resolve this problem for a slowly fading channel have involved the utilization of a function of the audio output in a feedback automatic gain control (AGC) operation to mute the receiver or otherwise temper the impact of the quiet time noise. Several examples are found in patents such as the K. Hagenhaus U.S. Pat. No. 2,179,928, the F. R. Skutta U.S. Pat. No. 4,013,964, the M. Ohsawa U.S. Pat. No. 4,121,161, and the J. Ben Sadou et al. U.S. Pat. No. 4,204,172. A common characteristic of these four references is that the gain control does not respond to the rapid fading experienced at microwave frequencies.